The present invention relates to the cryogenic separation of a gaseous mixture comprising carbon monoxide and hydrogen into its component gases. The present invention has particular application in the separation of gaseous mixtures further comprising methane to produce carbon monoxide and a fuel gas comprising hydrogen and methane.
There are many known processes for carrying out the cryogenic separation of gaseous mixtures comprising carbon monoxide and hydrogen. However, the difficulty confronting the gas production and separation industry is that a carbon monoxide-rich liquid that is used to provide refrigeration needs to be recycled in order to achieve an acceptable level of carbon monoxide recovery.
Once the carbon monoxide-rich stream has been used to provide refrigeration duty, it is usually compressed and recycled to the feed gas stream. For example, in DE-A-4210638 (xe2x80x9cFabian Ixe2x80x9d), a feed gas stream comprising carbon monoxide, hydrogen and methane is cooled and at least partially condensed by heat exchange and then separated into a hydrogen-rich stream and a carbon monoxide-rich stream comprising methane.
The hydrogen-rich stream is used to provide refrigeration duty by heat exchange against the feed gas stream and is further processed to produce a high purity stream of gaseous hydrogen. One portion of the carbon-monoxide stream has any remaining hydrogen stripped away and the resultant stripped stream separated into methane and high purity carbon monoxide. A further portion is used as a refrigerant stream to provide at least a portion of the refrigeration duty required to cool and at least partially condense the feed gas stream by heat exchange. The resultant vaporized refrigerant stream is then compressed and recycled to the feed gas stream.
U.S. Pat. No. 4,566,886 (xe2x80x9cFabian IIxe2x80x9d) discloses a further process in which a gaseous mixture of hydrogen and carbon monoxide is separated into a hydrogen-rich vapor and a carbon monoxide-rich liquid. A portion of the carbon monoxide-rich liquid is used to provide refrigeration duty for the feed gas. The resultant carbon monoxide-rich vapor is recycled to the feed stream and compressed to the feed gas pressure. A further portion of the carbon monoxide-rich stream is used to provide heat exchange duty and is then xe2x80x9crecycledxe2x80x9d to a hydrogen-stripping column. The overall compression power requirement for this process is not substantially different from that Fabian I.
The main drawback of most prior art processes (including the Fabian processes) is the requirement that the recycle carbon monoxide stream is compressed to the pressure of the feed gas stream. The power requirement for compression of a recycle stream to the feed pressure is responsible for a substantial part of the overall operating costs of a separation plant. It is the primary objective of the present invention to develop a process for separating carbon monoxide and hydrogen from a gaseous mixture thereof in which the overall compression power requirement is reduced thereby reducing the operating and capital costs of the separation plant without a significant loss in efficiency.
It has been found that by compressing carbon monoxide-rich vapor and recycling it to the hydrogen-stripping column, it is no longer a requirement that the recycled vapor be compressed to the pressure of the feed gas stream. Instead, carbon monoxide-rich vapor is compressed to a pressure below the feed pressure. Processes according to the invention show a significant reduction to the compression power requirement and, thus, the overall operating and capital costs of a separation plant to run a process according to the present invention are reduced.
In particular, feed gas comprising carbon monoxide and hydrogen is cooled and partially condensed by heat exchange to produce cooled and partially condensed feed gas which is separated to produce a first hydrogen-rich vapor and a first carbon monoxide-rich liquid. A first stream of the first carbon monoxide-rich liquid is fed to a hydrogen stripping column having an operating pressure below the feed pressure. Hydrogen is at least partially stripped from the carbon monoxide-rich liquid to produce hydrogen-stripped carbon monoxide liquid and hydrogen-enriched carbon monoxide vapor. A second stream of the first carbon monoxide-rich liquid or a stream derived therefrom is vaporized to provide at least a portion of the refrigeration duty required to cool and partially condense the feed gas by heat exchange and produce carbon monoxide-rich vapor. The process is characterized in that the carbon monoxide-rich vapor is compressed to produce compressed carbon monoxide-rich vapor at a pressure below the feed pressure. The compressed carbon monoxide-rich vapor is cooled and at least partially condensed to produce cooled and at least partially condensed carbon monoxide-rich vapor, at least a portion of which is recycled to the hydrogen stripping column.